Blood Physiology: Antigenicity and Immune Reactions

Antigenicity and Immune Reactions of Blood

• Early blood transfusions often led to immediate or delayed agglutination and hemolysis of red blood cells (RBCs), resulting in typical transfusion reactions that frequently led to death.
• The blood types of different people have different antigenic and immune properties, with antibodies in one blood type reacting with antigens on the surfaces of RBCs of another blood type.

Multiplicity of Antigens in Blood Cells

• At least 30 commonly occurring antigens and hundreds of rare antigens have been found on the surfaces of human blood cell membranes.
• Most antigens are weak and important for studying the inheritance of genes to establish parentage.
• Two main types of antigens that cause blood transfusion reactions are the O-A-B system and the Rh system.

O-A-B Blood Types

• Type A and type B antigens (agglutinogens) occur on the surfaces of RBCs in a large proportion of people.
• These agglutinogens cause most blood transfusion reactions.
• People can have neither A nor B agglutinogens, one, or both simultaneously due to inheritance.

Classification of O-A-B Blood Types

• Blood is classified into four major O-A-B blood types: O, A, B, and AB.
• Type O blood has neither A nor B agglutinogens.
• Type A blood has only type A agglutinogen.
• Type B blood has only type B agglutinogen.
• Type AB blood has both A and B agglutinogens.

Genetic Determination of Agglutinogens

• The ABO blood group genetic locus has three alleles: IA, IB, and IO.
• These alleles determine the three blood types: A, B, and O.
• The type O allele is functionless or almost functionless, while the type A and type B alleles cause strong agglutinogens on the cells.
• The O allele is recessive to both the A and B alleles, which show co-dominance.

Genotypes and Blood Types

• Each person has two sets of chromosomes, with one of the three alleles present on each chromosome.
• There are six possible combinations of alleles: OO, OA, OB, AA, BB, and AB.
• These combinations of alleles are known as the genotypes.
• Each genotype corresponds to a specific blood type:
• OO: O
• OA or AA: A
• OB or BB: B
• AB: AB